Presently, there are two known techniques that employ a DC control voltage to attenuate AC signals in the frequency band that extends from, approximately, 10 hertz to 30 megahertz. The first type of attenuator is the digital signal multiplier. The digital signal multiplier uses a DC control signal to control the reference voltage in a digital-to-analog (D/A) converter. The D/A converter, in turn, produces an output that is the product of the reference voltage multiplied by the digitized AC signal. Thus, by restricting the reference voltage to a normalized range extending from zero to one, attenuation of the AC signal can be achieved. The digitized AC signal that is input to the D/A converter must first be obtained by pre-processing the AC signal with an analog-to-digital (A/D) converter. Consequently, the digital multiplier attenuator requires both an A/D converter and a D/A converter. There are several drawbacks inherent in such attenuators. Namely, A/D and D/A converters are expensive and complicated. Further, D/A and A/D converters consume a great deal of area on a printed circuit board thereby reducing the functionality per unit area of the printed circuit board. Thus, there exists a need for an inexpensive attenuator that occupies a relatively small amount of printed circuit board area.
The second type of AC signal attenuator that utilizes a DC control signal is an analog multiplier. Typically, the analog multiplier uses a DC control signal to adjust the gain of a logarithmic amplifier. Adjustment of the gain of the logarithmic amplifier results in the desired AC signal attenuation. Exemplary of known analog multipliers is U.S. Pat. No. 3,714,462, issued to Blackmer on Jan. 30, 1973, for an invention entitled "Multiplier Circuits". Of related interest is U.S. Pat. No. 4,471,324 issued to Welland on Sept. 11, 1984, for an "All NPN Variably Controlled Amplifier". Current analog multipliers, however, possess several undesirable features. Namely, known analog multipliers possess bandwidths that are limited to frequencies below 1 megahertz. Consequently, such attenuators are not useful in the frequency band that extends from 1 megahertz to approximately 30 megahertz. Further, known analog multipliers, such as Blackmer, typically exhibit a high part count which increases the cost of the attenuator. Concomitantly, a high part count also increases the amount of printed circuit board area required for the attenuator and, hence, reduces the functionality per unit area of the printed circuit board. In addition, presently known analog multipliers require expensive, high precision parts to realize an attenuator that is both temperature independent and stable throughout its bandwidth. Further, as Blackmer demonstrates, current analog multipliers require a considerable amount of circuitry to realize linear operation. This also increases the cost, complexity and printed circuit board area needed to realize an attenuator. Therefore, there is a further need for an attenuator that exhibits an improved bandwidth, a reduced part count, and provides linear operation without the need for expensive, high precision parts.
Several attenuators which operate in the radio frequency (RF) band, extending from approximately 30 MHz to 10 GHz, and utilize a DC control signal are known. Exemplary of known attenuators which operate in the RF band and utilize a DC control signal is U.S. Pat. No. 4,646,036, issued to Brown on Feb.24, 1987, for a "Signal Attenuation Circuit". Brown employs a DC control signal to control two PIN diodes that provide variable attenuation of a radio signal.
Automatic gain control circuits, typically, maintain a constant output signal by attenuating or amplifying an input signal as required. Indicative of the automatic gain control circuits is U.S. Pat. No. 4,405,903, issued to Blackburn on Sept.20, 1983, for a "Variolosser For An Automatic Gain Control Circuit" which, apparently, employs a feedback loop to control a variable impedance diode. The feedback loop operates to maintain a constant input signal to a fixed gain amplifier. Consequently, the signal output by the fixed gain amplifier is also maintained at a constant level.